Considerable research has been conducted in the past to devise inorganic glasses exhibiting low transition temperatures (T.sub.g), thereby enabling melting and forming operations to be carried out at low temperatures. As conventionally defined, the transition temperature of a glass is that temperature at which a notable increase in the thermal expansion coefficient is recorded, accompanied by a change in specific heat (C.sub.p). More recently, it has been recognized that glasses demonstrating low transition temperatures are potentially useful materials for a host of applications, including applications as sealing materials and as a component in glass-organic polymer alloys and composites. Alloys, a very recent development disclosed in U.S. Pat. No. 5,043,369 (Bahn et al.), are prepared from a glass and a thermoplastic or thermosetting polymer having compatible working temperatures; i.e., the glass and the polymer are combined at the working temperature to form an intimate mixture. Articles produced from these alloy materials exhibit chemical and physical properties comprising a complementary blend of those demonstrated by the particular glass and polymer. For example, the alloys frequently display a combination of high surface hardness, high stiffness, and high toughness. Glasses having base compositions within the general zinc-phosphate system have been found to be especially suitable for the glass component of glass-polymer alloys. Two illustrations of recent research in this zinc-phosphate system are reported below.
U.S. Pat. No. 4,940,677 (Beall et al.) discloses glasses exhibiting transition temperatures below 450.degree. C., preferably below 350.degree. C., consisting essentially, in mole percent, of at least 65% total of 23-55% ZnO, 28-40% P.sub.2 O.sub.5, and 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions selected from the group of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, and up to 35% total of optional constituents. The optionals are selected from the group of 0-6% Al.sub.2 O.sub.3, 0-8% B.sub.2 O.sub.3, 0-8% Al.sub.2 O.sub.3 +B.sub.2 O.sub.3 , 0-15% CU.sub.2 O, 0-5% F, 0-35% PbO, 0-35% SnO, 0-35% PbO+SnO, 0-5% ZrO.sub.2, 0-4% SiO.sub.2, and 0-15% MgO+CaO+SrO+BaO+MnO, consisting of 0-10% MgO, 0-10% CaO, 0-10% SrO, 0-12% BaO, and 0-10% MnO.
U.S. Pat. No. 5,286,683 (Aitken) discloses a preferred composition consisting essentially of 30-35% P.sub.2 O.sub.5, 5-15% Na.sub.2 O, 5-10% Li.sub.2 O, 0-7% K.sub.2 O, 13-25% Li.sub.2 O+Na.sub.2 O+K.sub.2 O, 15-45% CuO, 0-15% CaO, 0-15% SrO, 0-15% BaO, 0-15% CaO+SrO+BaO, 0-3% Al.sub.2 O.sub.3, 0-3% B.sub.2 O.sub.3 0-3% Al.sub.2 O.sub.3,+B.sub.2, 0-30% Zn and 0-27% Sb.sub.2 O.sub.3. Furthermore, there is disclosed therein the requirement that a predominate portion of the copper present in the glass be in the Cu.sup.+2, or cupric oxidation state and that at least two alkali metal oxides be present.
The above-described zinc-phosphate glasses demonstrate relatively excellent resistance to chemical attack when compared to previous phosphate-based glasses. Nevertheless, the search has been continuous to discover new and different glass compositions displaying low transition temperatures with equivalent or even greater chemical durability. Resultant advantages of these lowered transition temperature, yet durable glasses, would include: (1) lowered energy costs attributed the glass formation and the subsequent preparation of glass/polymer alloys and composites; (2) an increase in the number of compatible polymers available for co-processing with the glass to form glass/polymer composites and for thermally co-deforming with the glass to form glass/polymer alloys; (3) a likely increase in the number of potential commercial applications of the alloys and composites; and, (4) uses of the glass as sealing materials.
Previously, an inherent drawback of lowering the transition temperature was the corresponding decrease in the durability, i.e., reduced resistance to attack by water. Accordingly, the principal objective of the present invention was to devise glass compositions having a transition temperature normally below about 300.degree., a working temperature below about 400.degree., while, at the same time, exhibiting excellent resistance to attack by water, i.e., at least comparable to glass compositions with much higher transition temperatures.